1. Field of Invention
The present invention relates to a piezoelectric single crystal and a fabrication method thereof. More particularly, the invention relates to a piezoelectric single crystal formed of a piezoelectric single crystal material developed based on an electromechanical coupling factor k31 in a direction orthogonal to the polarization direction.
2. Description of Related Art
For example, as shown in FIG. 1, with a rod-shaped piezoelectric single crystal device (L/a≧3, a=b) having an aspect ratio (L/a) of 3 or higher, of which the longitudinal direction is the polarization direction 3, the magnitude of vibration in the polarization direction 3 (the vertical vibration) at the time of applying a voltage along the polarization direction 3 is represented by an electromechanical coupling factor k33 in a longitudinal vibration mode that is proportional to the square root of the conversion efficiency of the electric energy and the mechanical energy. The greater this value is, the more the efficiency improves. Also, as shown in FIG. 2A, with a plate-shaped piezoelectric single crystal device (a>>L, b>>L) having an aspect ratio (a/b) of 2.5 or higher, the greater the value of the electromechanical coupling factor k31 in a direction 1 orthogonal to the polarization direction 3 (lateral vibration mode) is, the more the efficiency improves, as well.
It should be noted that, with a piezoelectric single crystal device, the electromechanical coupling factor of the device can be obtained in the same way with any wafer, such as, for instance, rectangular wafers, disc-shaped wafers, or the like, in addition to the above-described rod-shaped or plate-shaped wafer.
It should be noted that examples of the best known materials are piezoelectric single crystals comprised of a solid solution of lead zinc niobate Pb(Zn1/3Nb2/3)O3 and lead titanate PbTiO3 (referred to as PZN-PT or PZNT), or a solid solution of lead magnesium niobate Pb(Mg1/3Nb2/3)O3 and lead titanate PbTiO3 (referred to as PMN-PT or PMNT).
For example, Japanese Unexamined Patent Application Publication No. 6-38963 discloses an ultrasonic probe using a piezoelectric material comprised of a solid solution single crystal of lead zinc niobate-lead titanate (PZN-PT). This technique provides a probe with high sensitivity by using the single crystal of such a piezoelectric material which has an electromechanical coupling factor k33 of 80 to 85% in a direction parallel to the polarization direction 3, i.e., a vibration mode of the vertical direction 3. Conventionally, while the electromechanical coupling factors k33 in the direction parallel to the polarization direction 3 of the piezoelectric single crystals as described above have been studied and various usages have been developed, the properties in a direction orthogonal to the polarization direction 3 have not been studied yet.
It should be noted that the cases where piezoelectric devices have an electromechanical coupling factor k31 greater than 80% in the direction 1 orthogonal to the polarization direction 3 (lateral vibration mode) are described in the documents Jpn.J.Appl.Phys.41 (2002) L55 and Jpn.J.Appl.Phys.41 (2002) pp. 7108 to 7112 written by Ogawa et al.
However, neither of these documents mention that there is a relationship between the electromechanical coupling factor k31 in the direction 1 orthogonal to the polarization direction 3 (lateral vibration mode) and the domain structure, nor a repeatability exceeding 80%, and accordingly, this experimental data is assumed to have been accidentally obtained without repeatability, as in the case of other known documents.
The term “domain structure” generally means a striped pattern which can be observed on the surface of the device material with the naked eye or with a stereomicroscope as shown in FIGS. 7B, 7C, and 9B when a single crystal is cut out to a size suitable for a device. This striped pattern is a concentration pattern, and the intervals therein may be anywhere from several micrometers to about two hundred micrometers thick, and in some cases intervals of 1 mm or more are observed. Hereafter, in the present invention, “domain structure” should be understood to mean “the direction in which the stripes extend on the device surface”, unless indicated otherwise.